Imaging systems use luminescence to detect the presence of proteins, nucleic acids and other biological substances. Measuring the intensity of the luminescence will provide an indication of the concentration of the substance in a sample, for example, by comparing the intensity of the light among a plurality of samples or sample spots on a substrate.
When the luminescence is the result of excitation of the sample by an external excitation source (e.g., a light source), the luminescence or emission of light will typically be constant, and a useful dynamic range of detected light levels for comparing intensities can usually be obtained without difficulty.
In some cases, chemiluminescence may be a preferred technique for the detection of certain substances. In chemiluminescense, light is emitted from a sample based on a chemical reaction, resulting, for example, from the introduction of an enzyme and an oxidant to a sample of a protein or other substance of interest.
One limitation of chemiluminescence is that the emitted light tends to be faint, and it has a limited half-life and it degrades over time. As a result, it can be difficult to accurately determine the exposure time needed to get the best measurement of emitted light in an image that will provide a useful dynamic range of light levels. When using a photo detector device (such as a CCD camera), the exposure time should provide a dynamic range of light levels that capture enough light from sample spots giving off the lowest light intensity for meaningful evaluation, but without other spots emitting too much light and resulting in individual CCD elements becoming saturated due to over exposure. In the past, the exposure time of a chemiluminescent sample was often the result of a somewhat subjective estimate. The estimated exposure time does not always provided a good result and multiple measurements might need to be taken to obtain useful data.